The present invention relates to acoustic measurement, more particularly to method and apparatus for measuring the acoustic near field of a structure.
The term "acoustic near field" refers to the area of a structure where both radiating and non-radiating (i.e., evanescent) acoustic waves exist. The term "acoustic far field" refers to the area around a structure where only the radiating acoustic waves exist and the non-radiating acoustic waves have decayed.
For a given structure producing radiating and non-radiating waves, the radiating waves produce observable responses in the acoustic far field while the non-radiating acoustic waves do not. In mapping the acoustic near field of the structure, both the radiating and the non-radiating acoustic waves are measured. Using this technique, information on the types of structural waves traveling along the structure and their contribution to the observable responses in the acoustic far field can be obtained.
Three basic measurement approaches have been conventionally employed for mapping the acoustic near field of a structure, viz.: (i) moving the structure with respect to a stationary scanning system; (ii) moving a scanning system with respect to the stationary structure; and, (iii) surrounding the structure with a multitude of acoustic measurement devices which are included in the scanning system.
In accordance with the first two conventional approaches, the scanning system includes an array of acoustic measurement devices; either the structure is moved or the array of acoustic measurement devices is moved. Frequently, the array of acoustic measurement devices for these conventional techniques is a linear array or a planar array.
Application of the first approach to a larger structure (e.g., a structure occupying a rectangular region of the size 30'.times.30'.times.180') may be impractical, as moving the structure may become unwieldy; moreover, difficulties may arise in maintaining the appropriate distance between the acoustic measurement devices and the moving structure.
Application of the second approach has also had its practical difficulties for larger structures. The conventional technique of mounting acoustic measurement devices on a computer-controlled robot arm can readily measure the acoustic near field of a small structure (e.g., a structure occupying a rectangular region of the size 0.5'.times.0.5'.times.3.0'); however, the same robot arm may be found to be inappropriate for a larger version of the structure, and the expense and effort for building a scaled-up version of the robot arm may be prohibitive.
According to the third conventional technique, the scanning system includes acoustic measurement devices located at all positions where measurements of the acoustic near field are desired. The advantage of this technique vis-a-vis' the first two conventional techniques is that neither the structure nor the acoustic measurement devices need be moved; however, this third conventional technique often requires implementation of numerous acoustic measurement devices, which may be forbiddingly costly. Furthermore, utilizing this technique so as to place acoustic measurement devices at numerous locations and thereby conduct a "full array" covering of a structure has been known to result in distortion of the acoustic near field being measured.
Clark and Sartori at U.S. Pat. No. 5,347,496, issued 13 Sep. 1994, incorporated herein by reference, disclose mapping of the acoustic near field of an axially symmetrical structure whereby the structure is subjected to excitation energy along one or more lines which are longitudinally conformal with the surface of the structure.